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The Photochemical Irradiation of R3SiH in the Presence of [(5-C5H5)Fe(CO)2CH3] in DMF Leads to Disiloxanes not Disilanes.

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Zuschriften
DOI: 10.1002/ange.200903021
Disiloxanes
The Photochemical Irradiation of R3SiH in the Presence of
[(h5-C5H5)Fe(CO)2CH3] in DMF Leads to Disiloxanes not Disilanes**
Hemant K. Sharma and Keith H. Pannell*
We recently reported that the photochemical irradiation of
tBu2SnH2 in the presence of a catalytic amount of [(h5C5H5)Fe(CO)2CH3] ([FpMe]) led to the efficient synthesis of
the distannane HtBu2SnSntBu2H in hydrocarbon solutions
[Eq. (1)].[1]
2 tBu2 SnH2
hn
!
½FpMe;C6 D6
HtBu2 SnSntBu2 H þ H2
21.5 ppm.[3] Figure 1 illustrates this clean process for silane
1.
The GC–MS analysis of the product of this reaction also
confirmed the formation of the disiloxane and the absence of
ð1Þ
It was also recently noted that a similar irradiation of the
tertiary silanes R3SiH with the same catalyst [FpMe] led to
the high-yield formation of the corresponding disilane
R3SiSiR3, but only in the presence of N,N-dimethylformamide
(DMF) as solvent [Eq. (2)].[2]
2 R3 SiH
hn
!
½FpMe;DMF
R3 SiSiR3 þ H2
ð2Þ
The ability of DMF to react with the hydrogen formed in
this dehydrocoupling process to produce Me3N was used to
rationalize such interesting chemistry.
In our hands, the irradiation of the tertiary silanes R3SiH
reported in reference [2] (e.g. Ph2MeSiH (1), PhMe2SiH (2),
Et3SiH (3)) in the presence of the catalyst [FpMe] in DMF
solution results in the high-yield formation of the corresponding disiloxanes R3Si-O-SiR3 and no apparent formation of the
disilane.
In our experiments, 130 mg samples of the tertiary silanes
(Gelest) were irradiated in dry DMF (Aldrich) with 5 mol %
of the catalyst [FpMe] in sealed NMR tubes at a distance of
3 cm from a 400 W low-pressure mercury lamp. The reactions
were monitored by 29Si NMR spectroscopy. The reactions
with 1, 2, and 3 behaved similarly, with the disappearance of
the resonance of the starting silane (d = 17.5 ppm (1) in
48 h; d = 17.2 ppm (2) in 1.5 h; d = 0.5 ppm (3) in 10 h)
along with the appearance of a single resonance associated
with the formation of the corresponding disiloxane
R3SiOSiR3 (d = 9.1, 0.6, and 8.4 ppm for the reactions of 1,
2, and 3, respectively). No resonances associated with the
corresponding disilanes were observed, for example
(Ph2MeSi)2 at d = 22.6 ppm[3] or (PhMe2Si)2 at d =
[*] Dr. H. K. Sharma, Prof. K. H. Pannell
Department of Chemistry, University of Texas at El Paso
El Paso, TX 79968-0513 (USA)
E-mail: kpannell@utep.edu
[**] We thank Dr. Shizue Mito for useful discussions and Dr. Ernesto
Nakayasu and the U. T. El Paso Border Biomedical Research Center
facility for access to, and training on, the GC–MS instrumentation.
We also thank the Welch Foundation, Houston, TX for support of
this research.
7186
Figure 1. 29Si NMR spectroscopic monitoring showing the disappearance of silane 1 in the presence a 5 mol % [FpMe] catalyst in DMF.
any disilanes. We illustrate the gas chromatograph of
PhMe2SiOSiMe2Ph along with that of a commercial product
(Gelest) in Figure 2. The equivalence is clear, with retention
Figure 2. GC retention time data for PhMe2SiOSiMe2Ph, synthesized
herein (top) and an authentic sample (Gelest; bottom).
times of 10.14 and 10.16 min, respectively. The small amount
of material at a retention time of 11.28 min is the trisiloxane
PhMe2SiOSiMe2OSiMe2Ph, which curiously is present both in
the commercial material and the material synthesized in our
experiments.
The mass spectral portion of the GC–MS analysis of the
three siloxane materials formed in our studies are illustrated
in Figure 3. The inverted mass spectrum on the bottom of
each part of Figure 3 is that derived from the standard library
of materials and demonstrates absolute equivalence of the
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2009, 121, 7186 –7188
Angewandte
Chemie
exposed the appropriate disilane to the same photochemical
conditions, in the same solvent, either dry or used “as
received”, and detected no disiloxane production [Eq. (3)].
PhMe2 SiSiMe2 Ph
Figure 3. Mass spectra of a) Ph2MeSiOSiMePh2, b) PhMe2SiOSiMe2Ph,
and c) Et3SiOSiEt3 formed in the reactions presented (top) and the
literature spectra (bottom).
product to the reported standards.[4] No remaining R3SiH, nor
any disilane R3SiSiR3, was detected. In the report on the
formation of disilanes,[2] neither structural nor spectroscopic
evidence of the disilanes was reported, only GC analyses. In
our hands, the use of GC to distinguish the disilane from the
disiloxane product resulted in retention times of 10.14 min for
PhMe2SiOSiMe2Ph and 10.35 min for PhMe2SiSiMe2Ph
(Gelest) under our instrumental conditions. We note that in
the supporting information associated with reference [2], the
reported elemental analyses correspond more closely to
disiloxane products than disilanes. Furthermore, the chemical
shift in the 29Si NMR spectrum presented for poly[(tetramethyldisilanylene)ferrocenylene]
[{(h5-C5H4)Fe(h5-C5H4SiMe2SiMe2)}n] was d = 0.78 ppm, close to that of the model
disiloxane complex [{(h5-C5H5)Fe(h5-C5H4)}2SiMe2OSiMe2],
which we reported at d = 0.48 ppm.[5] In contrast, we also
reported a model disilane complex of this type, that is, [{(h5C5H5)Fe(h5-C5H4)}2SiMe2SiMe2], and noted that its 29Si NMR
spectrum contained a single resonance at d = 22.4 ppm.[6]
Thus we have clearly demonstrated that only disiloxane
formation takes place under the photochemical conditions
reported above.[2]
We performed a series of experiments to determine
whether any variation of the reaction conditions could
account for our observations. For example, reactions with
either dry or wet DMF yielded the same results. We also
Angew. Chem. 2009, 121, 7186 –7188
hn
!
½FpMe;DMF
no reaction
ð3Þ
This result rules out initial formation of the disilanes and
subsequent oxidation. At present we are unable to identify
the specific features of our experimental setup that could
result in the production of the siloxanes as opposed to
disilanes.
The ability of DMF to remove the hydrogen produced to
form Me3N was suggested as a crucial feature for the solventspecific process; however, this provokes the question as to the
fate of the oxygen atom. In 1985 Voronkov and co-workers
reported that treatment of various silanes R2R’SiH (R2R’ =
Cl2Me, Cl2Et, Et2Me, and Et3) with DMF in the presence of
metal species (NO)2PtCl6 or [Me2NH2][Rh(CO)2Cl2] led to
the formation of the corresponding disiloxanes.[7] In that
study, and from the product yield/time relationship of the
recent “disilane” synthesis, it was concluded that electronwithdrawing groups slowed down the process. We can confirm
the same relationship using 1, 2, and Ph3SiH. Indeed, in our
hands the photolysis of Ph3SiH with [FpMe] failed to yield a
significant amount of Ph3SiOSiPh3, contrary to the reportedly
slow reaction but high recovered yield of Ph3SiSiPh3.[2]
As to a mechanism for this interesting new chemistry,
there are several possibilities. One certainly involves the
formation of a bis(silyl) iron complex originally proposed for
the formation of the disilanes.[2] However, the fact that
disiloxane formation was noted by Voronkov using metal
complexes of Pt and Rh, for which bis(silyl) complexes are
less obviously accessible leads us to suggest the mechanism
outlined in Scheme 1.
The key feature of this mechanism involves formation of
the DMF metal complex and a subsequent hydrosilylation–
reductive elimination process. There is significant precedent
for the photochemical formation of DMF metal carbonyl
complexes,[8] in which the electron-deficient ketone carbon
atom is activated towards hydrosilylation chemistry.[9]
As noted in Scheme 1, it is possible that the hydrosilylated
product R3SiOCH2NMe2 could be eliminated and itself react
with R3SiH to form the disiloxane and Me3N. This suggestion
comes from a related precedent from Mironov and coworkers involving disiloxane elimination reactions of this
product with chlorosilanes and related species [Eq. (4), X =
Cl, NR2 etc.].[10]
Me3 SiOCH2 NMe2 þ Me3 SiX ! Me3 SiOSiMe3 þ XCH2 NMe2
ð4Þ
We are continuing a detailed mechanistic study of this
system, including studies of R3SiOCH2NMe2 compounds.
Regardless of the exact process, this catalytic process may be
very general, as exchanging [FpMe] for [(h5C5H5)Mo(CO)3CH3] and other related complexes accomplishes the same transformation of silane to disiloxane.[11] A
recent discussion on the role of DMF in activating the
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
7187
Zuschriften
Scheme 1. Proposed catalytic mechanism for transformation of R3SiH to R3SiOSiR3.
dehydrocoupling of silanes with hydroxylic reagents illustrates the complexity of the DMF–silane interactions.[12]
Received: June 4, 2009
Published online: August 22, 2009
[4]
.
Keywords: homogeneous catalysis · iron · oxygenation · silanes
[1] a) K. H. Pannell, H. K. Sharma, R. Arias, Abstracts of Papers,
235th ACS National Meeting, New Orleans, LA, United States,
April 6 – 10, 2008, INOR-015; b) H. K. Sharma, R. Arias-Ugarte,
A. J. Metta-Magana, K. H. Pannell, Angew. Chem. 2009, 121,
6427; Angew. Chem. Int. Ed. 2009, 48, 6309.
[2] a) M. Itazaki, K. Ueda, H. Nakazawa, Angew. Chem. 2009, 121,
3363; Angew. Chem. Int. Ed. 2009, 48, 3313; b) “Disilane bondcontaining polymers and preparation thereof”: H. Nakazawa, M.
Itazaki, Jpn. Kokai Tokkyo Koho, 2009; c) “Preparation of SiSi
bond-bearing compounds”: H. Nakazawa, M. Itazaki, U.S. Pat.
Appl. Publ., 2009.
[3] 29Si chemical shift in the NMR spectra of a commercial sample
(Gelest) of PhMe2SiSiMe2Ph in DMF (without lock) is
7188
www.angewandte.de
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
observed at d = 21.8 ppm and in C6D6 at d = 21.48 ppm. 29Si
chemical shift of the disilane Ph2MeSiSiMePh2 synthesized
from equimolar amounts of Ph2MeSiLi and Ph2MeSiCl in THF is
observed at d = 22.56 ppm in C6D6.
GC–MS analysis was performed using a trace GC connected to a
Polaris Q mass spectrometer (Thermo Fisher Scientific). The
library search algorithm used was the NIST MS Search 2.0,
which comes in Xcalibur 1.4 SR1 (Thermo Fisher Scientific)
package.
K. H. Pannell, H. K. Sharma, Organometallics 1997, 16, 3077.
K. H. Pannell, H. Sharma, Organometallics 1991, 10, 954.
L. I. Kopylova, N. D. Ivanova, M. G. Voronkov, Zhur. Obshch.
Khim. 1985, 55, 1649.
I. W. Stolz, G. R. Dobson, R. K. Sheline, Inorg. Chem. 1963, 2,
323.
a) S. Diez-Gonzlez, S. P. Nolan, Organic Preparations and
Procedures International 2007, 39, 523; b) R. H. Morris, Chem.
Soc. Rev. 2009, 38, 2282.
V. P. Kozyukov, V. P. Kozyukov, V. F. Mironov, Zh. Obshch.
Khim. 1983, 53, 119.
R. Arias, unpublished results.
J. J. Chruściel, Can. J. Chem. 2005, 83, 508.
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2009, 121, 7186 –7188
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presence, 2ch3, dmf, leads, disilane, r3sih, irradiation, disiloxanes, photochemical, c5h5
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